Circuit arrangement for measuring the current consumption of a transistor-controlled load

ABSTRACT

The invention relates to a circuit arrangement for measuring the current consumption of a transistor-controlled load, in particular of an electromotor, whereby a load current (I L ) is detected by measuring the controlling current (I T2 ) of an output bipolar transistor (T1). The controlling current (I T2 ) is detected using the voltage drop across an internal measuring shunt (R 1 ). As the controlling current (I T2 ) is substantially smaller than the load current, the power losses are negligible in comparison with a measuring shunt that is connected in series to the load. In addition, there is no need for a controlling unit input, to which an external measuring-circuit voltage is applied.

[0001] The invention pertains to a circuit array for measuring the current consumption of a transistor-controlled load, in particular of an electric motor. It is the purpose of such circuit arrays to detect the current in a load circuit, in order to allow the load to be controlled. The current is usually measured by a precision resistor seriate relative to the load, so that a measuring voltage proportional to the current in accordance with the relation U=R * I can be tapped over the resistor. It is a problem of such an array that, in the precision resistor, a relatively great output is converted into heat because the precision resistor must not be too small for a sufficiently high measuring voltage to be obtained for evaluation. Especially when large currents are present in the load circuit, this problem attains greater significance, as the power dissipation rises quadratically with respect to the load.

[0002] It is therefore the purpose of the invention to describe a circuit array in which the power dissipation caused by measuring the load current is very small, but wherein the design of said circuit array is nevertheless simple.

[0003] This purpose is met by way of a circuit array with an output bipolar transistor switched as a voltage follower, with a controlling device for controlling the base of the output transistor, with a precision resistor, and with a measuring device which detects the voltage drop over the precision resistor. In this array, the precision resistor is located in a branch parallel to the base-collector segment of the output transistor.

[0004] The circuit array in the invention takes advantage of the fact that, when a transistor is switched as a voltage follower, the control current at the base is proportional to the emitter collector current, i.e. the load current. The control current and the load current are coupled via the forward amplification, so called, of the output transistor, which is nearly constant over a wide area. A first great advantage arises from the fact that the power dissipation converted in the precision resistor is lower by a factor of 10 to 50 than in an array in keeping with the state of the art. The resistor may therefore safely be integrated into a controlling unit which, for example, is embodied as an integrated circuit. Another advantage arises from the fact that it permits the realization of a component containing the controlling unit with one less pin, as the pin to which the measuring voltage is otherwise fed is rendered superfluous.

[0005] In an advantageous embodiment of the invention, an evaluative device connected to the device which measures the current evaluates impulses present in the controlling current of the output transistor. This is of advantage because, as a rule, the forward current amplification is not precisely known. During evaluation of impulses, in which the time of their occurrence is frequently the only factor of interest, the absolute value of the voltage or the current is of no importance.

[0006] Additional details and embodiments of the invention are described in the sub-claims.

[0007] Below, the invention is explained in detail by reference to an embodiment.

[0008]FIG. 1 shows a circuit array in accordance with the state of the art, and

[0009]FIG. 2 shows a circuit array in accordance with the invention.

[0010]FIG. 1 shows a circuit array in accordance with the state of the art, used for driving a Ventilated Motor 1. The Ventilator 1, an Output Transistor T1 and a Precision Resistor R_(S) are switched in series between an Operating Voltage Source U_(B) and the Reference Potential 0. Thus, Precision Resistor R_(S) is arrayed between Transistor T1 and Reference Potential 0. The voltage over Precision Resistor R_(S) is tapped and fed to a Measuring Port 2 of a Controlling Unit 3. Transistor T1 is arrayed as a voltage follower. As a result, voltage amplification is approximately 1 and current amplification typically ranges between a factor of 10 and 50. The base of Output Transistor T1 is connected to a Controlling Output Port 4 of Controlling Unit 3. The design of Controlling Unit 3 essentially comprises a Controlling Transistor T2 which controls the base current of Output Transistor T1, and a Servo Amplifier 5, which controls Controlling Transistor T2 on the basis of the voltage value fed to Measuring Port 2. An additional Operating Voltage U_(st) may be fed to a Controlling Input 6, presetting a desired value for driving Ventilated Motor 1.

[0011] Since Load Current I_(L) is relatively high, current dissipation in Precision Resistor R_(S) is likewise relatively high, so that the latter cannot be incorporated in an integrated circuit containing Controlling Device 3, due to the heat being generated.

[0012] A circuit array in accordance with the invention is illustrated in FIG. 2. The intended purpose of the circuit array is the same as in the state of the art illustrated in FIG. 1, i.e. the circuit array is used for controlling a Ventilated Motor 1. Output Transistor T1 does not differ from the one shown in FIG. 1. However, the collector of Transistor T1 is not connected to Reference Potential 0 via a precision resistor but directly fed to Reference Potential 0. As in the state of the art, the base of Output Transistor T1 is connected to a Controlling Output Port 4 of a Controlling Device 13. In order to determine Load Current I_(L), the circuit array of the invention evaluates the Controlling Current I_(T2) of Transistor T1. For this purpose, a Precision Resistor R_(I) is arrayed in the input path. Voltage over Resistor R_(I) is now a measure for the quantity of Controlling Current I_(T2). If forward amplification h_(FE) of Output Transistor T1 is known, Load Current I_(L) may be computed by multiplying Controlling Current I_(T2) with forward amplification h_(FE). In the instant case, however, it is not so much the absolute value of the load current that is of interest as rather the times of commutation. It is therefore sufficient for a current to flow through Precision Resistor R_(I) which is proportional to the load current. It may be assumed that minimum forward amplification amounts to approximately 10. Output dissipation in Precision Resistor R_(I) is computed P_(tot)=I_(T2) ²* R_(I). As a result, current dissipation in Precision Resistor R_(I) is significantly less than in Precision Resistor R_(S) in accordance with the state of the art.

[0013] Voltage over Resistor R_(I) is tapped and fed to a Differentiator 16, since it is the frequency of the commutation impulses which is to be evaluated. The differentiator is connected to a Servo Amplifier 15, which thus receives a signal with pulses appropriate to the frequency of the commutation impulses.

[0014] Servo Amplifier 15 in turn controls Controlling Transistor T2. At one Control Input 6 of Servo Amplifier 15, a Desired Frequency f_(st) is provided, by which a desired frequency is preset for controlling Ventilated Motor 1.

[0015] Desired Frequency f_(st) is compared with the measured frequency made available by Differentiator 16, and Output Transistor T1 is controlled on the basis of the comparison. Instead of a Desired Frequency f_(st), a desired voltage may be provided, with which a voltage-controlled oscillator is then activated, allowing a desired frequency to be set in turn.

[0016] Due to the low power dissipation in Resistor R_(I), the latter may be integrated in Controlling Unit 13. An additional input serving to feed a measuring voltage is thus rendered superfluous. Saving one pin for a measurement input may markedly reduce the cost of manufacturing the controlling device.

[0017] In another embodiment, an npn transistor is used as output transistor. Appropriately, the polarities of distribution voltage are reversed in such an embodiment. The variant described above, with a pnp transistor has the advantage that Ventilator 1 can be driven with 12 V, whereas the controlling circuit is supplied with 5 V. When the polarities are reversed and an npn transistor is used, this ceases to be possible.

[0018] For the pnp transistor variant, the controlling transistor is an n-channel-FET, whereas a p-channel-FET would have to be used in the case of an npn transistor. However, N-channel-FETs are generally preferred because of their greater charge carrier mobility. 

1. Circuit array for measuring the current consumption of a transistor-controlled load, in particular of an electric motor, with one Output Bipolar Transistor (T1) arrayed as a voltage follower, a Controlling Unit (13) for activating the base of Output Transistor (T1), a Precision Resistor (R_(I)) located in a branch parallel to the base-collector segment of Output Transistor (T1), and a measuring device which detects the voltage drop over Precision Resistor (R_(I)) and which is connected to the controlling unit.
 2. Circuit array in accordance with claim 1, characterized by the fact that Output Bipolar Transistor (T1) is a pnp transistor whose collector is connected to a Reference Potential (0) and whose emitter is connected to the Load (1), the other Load (1) connector being connected to a Supply Potential (+U_(B)).
 3. Circuit array in accordance with claim 1, characterized by the fact that Load (1) is an electronically commuted electric motor and that Controlling Unit (13) is provided with an Evaluative Device (16) which evaluates impulses found in the voltage over Precision Resistor (R_(I)) for the purpose of determining the number of revolutions of Electric Motor (1).
 4. Circuit array in accordance with claim 1, characterized by the fact that Precision Resistor (R_(I)), the measuring device and Evaluative Device (16) are part of Controlling Unit (13) and are arrayed in a joint component case.
 5. Circuit array in accordance with one of claims 1 to 4, characterized by the fact that a Controlling Transistor (T2) is arrayed between a first terminal of Resistor (R_(I)) and Bipolar Output Transistor (T1), Controlling Transistor (T2) constituting part of Controlling Unit (13), and that the other terminal of Precision Resistor (R_(I)) is connected to the collector of Output Transistor (T1).
 6. Circuit array in accordance with claim 5, characterized by the fact that Controlling Transistor (T2) is a field effect transistor.
 7. Circuit array in accordance with claim 5, characterized by the fact that a Servo Amplifier (15) is arrayed between the first terminal of Precision Resistor (R_(I)) and a control terminal of Controlling Transistor (T2), Servo Amplifier (15) activating the Controlling Transistor (T1) based on the frequency of the measured impulses.
 8. Process for measuring the current consumption of a transistor-controlled load, in particular of an electric motor, with a Bipolar Output Transistor (T1) switched as a voltage follower, wherein the determination of current consumption of Load (1) is made by measuring Controlling Current (I_(T2)) of Bipolar Output Transistor (T1).
 9. Process in accordance with claim 8, characterized by the fact that impulses found in Controlling Current (I_(T2)) are evaluated. 